1. Field of the Invention
The present invention relates to a moving object monitoring system for monitoring a moving object by using a leak transmission path such as a leaking cable or the like.
2. Description of the Related Art
According to a technique disclosed in JP-A-10-95338 (FIG. 1 and the description thereof), an electrical wave emitted from a transmission leaking cable is received by a reception leaking cable to monitor the level of this reception signal. When an object invades into the gap between the transmission and reception leaking cables, the electrical wave to be observed varies because it is delayed by the amount corresponding to the propagation delay time of an electrical wave reflected from the object, and thus the level of the reception signal is lowered. The level of the reception signal is determined on the basis of a predetermined threshold value at all times, and when the level of the reception signal is lower than the predetermined threshold value, an alarm for indicating existence of an invading object is output.
Furthermore, JP-A-2004-306909 (FIGS. 1 to 3 and the description thereof) discloses a system for detecting invasion of an object into the gap between leaking cables by using a spread spectrum signal.
In these systems, the distance in the perpendicular direction to the cable longitudinal direction of the leaking, that is, the separation distance cannot be measured, and thus an object which sufficiently reflects an electrical wave is detected although it is located far off. That is, these systems have a problem that the separation distance from the leaking cable cannot be used as a determination criterion.
The position of an object is calculated on the basis of only the propagation delay time from the emission of an electrical wave from the transmission leaking cable to reception of the electrical wave at the reception leaking cable, and thus the position of the object is calculated as a distance along the leaking axial cable. Accordingly, there is a problem that an alarm of “an invading object exists at a position on the leaking coaxial cable” is erroneously emitted because a large-size object such as a vehicle or the like moving in an extension direction (longitudinal direction) of the leaking cable at a position which is far away from the leaking coaxial cable in a direction perpendicular to the longitudinal direction (extension direction) of the leaking coaxial cable is detected.
Furthermore, even in the case of a small object, the reception level is greatly varied when it is very close to the leaking cable, and thus the small object is erroneously detected. That is, the size of the object cannot be used as a determination criterion for alarm.